I was just reading a hackaday article about this and thought it might be of interest to people here. Natalie the Nerd has successfully reverse engineered the gba main board.
You can read more and find a pcbway link to order on the github page:
https://github.com/nataliethenerd/AGB-CPU-03
You can read more and find a pcbway link to order on the github page:
https://github.com/nataliethenerd/AGB-CPU-03